U.S. Pat. No. 4,056,764 describes a power supply system with two batteries which are connected in parallel and which have a different characteristic and a power-limiting circuit which is connected upstream of a battery.
U.S. Pat. No. 5,316,868 describes a circuit for electrically connecting a primary battery and a secondary battery to a load and to a control unit which connects said secondary battery into the circuit if additional electrical power is required.
EP 0 363 356 B1 discloses a voltage supply system with a d.c./d.c. transformer for electronic devices, in particular, for safety-related devices in motor vehicles in which groups of devices, or each individual device, is assigned a separate supply circuit with an independent auxiliary voltage source which maintains the power supply of the associated electrical device if there is a fault in the supply circuit.
EP 0 398 691 B1 discloses a power distribution system in which an electrical connection is formed between a first and a second power source for a period of time as a function of the presence or absence of physical states, for example, a driver getting into a vehicle.
U.S. Pat. No. 5,844,325 describes a multiple battery system and its control in which a switch enables the same poles of two batteries to be connected to one another and a second switch permits the supply to auxiliary loads to be interrupted.
Publication DE 19855245 A1 describes a redundant voltage supply for electrical loads in a vehicle-mounted electrical system, in particular, for brakes which can be actuated electrically, having a generator which supplies two separate voltage branches which are d.c. isolated by means of a d.c. voltage transformer and can each be connected to their own voltage store and to the electrical loads, switching elements which are normally in the conductive state being located between each battery and the associated voltage branch, and between each battery and the load.
DE 19951128 A1 discloses a method for regulating the voltage in a vehicle-mounted electrical system with two batteries which can be charged by a generator and which are used to supply loads and can be connected to one another by means of a voltage transformer of a control device, it being possible to supply the control unit with information which is typical of the engine or vehicle-mounted network and is processed by a microcomputer, and to adapt the charge power for the starter battery in terms of timing to fluctuating power demands of electrical loads and to power outputs of the generator which fluctuate as a function of the engine speed, and to the capacity utilization of the generator.
In these known power supply systems with two batteries, it is impossible or possible only to a limited degree, to operate a targeted battery management system to avoid system faults due to inadequate charging and battery failures, and to ensure the emergency operating capability.
It would accordingly be advantageous to provide a power supply system having at least two electrical energy stores, which power supply system has improved reliability and availability by virtue of the possibility of battery management and redundancy in the case of a battery failure.